Removable adhesives are increasingly being used in many specialized tape and label applications. Adhesives are said to be removable when the tape or label structure can be removed without facestock failure or without significant residue being left behind after removal. These adhesives are comprised of many different chemistries and carriers. Many different performance profiles are needed due to the large number of application conditions, facestocks used, and end use substrates on which they are applied.
Because the adhesion level of these materials is engineered to be very low, anchorage (or adhesion) to the facestock can be a problem. This is generally overcome by the use of a primer coating which adds cost and complexity to the structure. Some facestocks can be purchased pre-primed with a higher energy coating. Many times, a primer is coated and dried on the facestock, either in-line with the adhesive coating, or as a separate pass on the coating equipment. Some of the most effective primers are permanent PSAs; however, this approach adds another additional layer of complexity. If the primer is tacky, it cannot be rewound in a self-wound manner. Another means of priming is to coat a non-pressure sensitive material on a facestock that has exceptional specific adhesion to the removable adhesives. This is usually the approach taken when the primer has to be applied in a separate pass and a primed face stock rewound prior to coating the adhesive. Often these are materials that are very similar to the chemistry of the adhesive. Furthermore, when the primer is applied to a porous face stock in a fluid state, it can flow into the facestock to some degree and when set, can form very strong bonds due to entanglements. Hence, anchorage is improved. Either way, the cost and complexity of the system is affected by the use of a primer to promote adhesion to the facestock.
The effectiveness of a removable adhesive can be evaluated by examining the residues left on stainless steel panels after performing a peel test on adhesives that were coated onto primed and unprimed facestocks. Samples may be heat aged (for example, at 50 C for one week before peeling) to provide more comprehensive data. Under such conditions, comparing primed and unprimed systems, the unprimed system shows substantial adhesive residue on the stainless steel panel indicating significant transfer of the adhesive from the facestock where it was designed to remain, to the substrate to which it had been applied. Removability of the primed system is evidenced by the essential lack of adhesive residue on the panel.
To further illustrate the need for priming, two different adhesives aged on a stainless steel substrate a function of time. Even though there is a lower peel and higher peel adhesive, both transfer to the substrate regardless of dwell time. As can be seen in Table 1, priming is an effective means of improving anchorage.
TABLE 190° Stainless Steel Peels (oz./in.) of Primed and Unprimed Adhesives1 Week20 Min24 Hr1 Week50° C.ROBOND PS-8120HV3.6AFB4.4AFB5.8AFB6.4AFBunprimedROBOND PS-8120HV6.0A7.4A6.3A8.2Aprimed with RHOPLEX P-376ROBOND PS-89316.8AFB8.9AFB11.0AFB6.8AFBunprimedROBOND PS-893113.3A16.4A15.3A12.9Aprimed with P-376A = Adhesive Failure, AFB = Adhesive from Backing failure.
In cases where the facestock of interest is porous, such as paper facestocks, improved anchorage could be achieved with just a single pass adhesive, if the adhesive would flow into the pores. Removable adhesives typically have very high cohesive strength and relatively little cold flow, so when they are transfer coated, only a very minor portion will flow into the pores. One approach to alleviate this problem is to add material that will flow into the pores while coating, yet have very little cold flow in its end use. Materials that are either thermoplastic or display some crystallinity have the potential of being of lower viscosity in a molten phase, yet rigid when in a glassy or crystalline state. Since the coating of most PSAs requires heat, it is possible to identify a material that undergoes a phase transition between the temperature a web would reach during the coating process and ambient conditions. Moreover, it is highly desirable that this material remain as a separate phase from the bulk of the adhesive if this desired phenomenon is to be maintained.
To examine this phenomenon in more detail, samples with and without a thermoplastic filler (35 wt. %) were prepared and transfer coated onto paper by sending the laminate through a hot nip. Afterwards, cross-sections were prepared by a cryosectioning and the surfaces were imaged by scanning electron microscopy. Since the filler is a relatively molten, low-viscosity material when nipped, it is more apt to flow into the pores of the paper. In a micrograph, this difference appears to be visible as the rougher interface, as compared to systems without the thermoplastic filler. Additional testing showed that anchorage was clearly improved with the addition of the filler.
Historically, PSA formulations have been identified that achieve this effect by adding wax to the adhesive; for example, Blackwell, U.S. Pat. No. 5,073,457 (assigned to 3M). Blackwell taught blending of a petroleum wax (paraffin wax or a polyethylene or polypropylene was emulsion) with a solvent-based or water-based acrylic adhesive. In Blackwell, relatively low levels of wax were used and the patent describes improved anchorage which was speculated to be associated with flow of the wax into pores in the substrate. Waxes are of low surface energy and low viscosity when in the molten form. Consequently, they can be added at low levels to reduce adhesion, since thermodynamically, they should be driven to the surface. This could also limit how much material is available to flow into pores of the substrate because higher levels of wax might overwhelm the surface of the adhesives to such an extent that too much adhesion would be lost.
The present invention provides a cost-effective and easy to manufacture primerless, removable PSA system which further provides improved balance of cohesive integrity within the system and adhesive performance of the resulting system. The use of C5-based tackifiers or other suitable thermoplastic fillers further provides a more controllable system to balance those properties.